[Dear Katerina,]
The exact answer to this question is a mix up made by the bishop of Prague. In his 1544 chronicle he listed ~Vlad Dracula~ as the ruler of Transylvania.

And now a couple of more info so that you`ll get the exact picture.
As already explained in my series about the historical Dracula, he wasn`t exactly loved by the merchants in the neighboring Hungarian cities of Brasov and Sibiu situated in… you guess it!!! … TRANSYLVANIA!
This was mainly because Vlad pretty much closed down the trade with them by creating 3 trade centers on the border with Wallachia and essentially told them not to “export” to his country. All their products were to be sold in large quantities and at fair prices so that the wallachian traders could do the rest [and the profit].
Needless to say this was not viewed kindly by the transylvanian barons and they started to write their king about “the evil Vlad Dracula”!
Enraged by this action, the said hungarian traders started to house rivals to Vlad`s throne in Brasov and Sibiu. Rather than making him more controlable, Vlad Dracula charged the regions and plundered them as punishment.
That is why we currently have so many bloody legends about Dracula.
Another thing that you might want to ponder on is that fact that in later part of the XV century the printer was barely invented! Despite it being such a rare and costly machine to employ, by the 1500s the whole of Europe was filled with brochures about the “odious” Vlad the Impaler!
So much money spent to make sure that Dracula wouldn`t get any support from any of the royal courts and to give Corvinus as excuse to keep him prisoner! 😉
Here`s a little list of the years so you understand how many brochures we are actually talking about and how long it took them to shape the myth:
– 1477: the oldest, appeared in the last year of his life and a copy is kept from the Lubeck edition
– 1491 in Bamberg
– 1494 in Augsburg
– 1499 in Nurnberg: we find one such edition in the collection of the humanist Hartmann Schedel
– 1520 in Strasbourg: the variations are so bizarre that this format barely resembles the original from 1477.
In conclusion: He was associated with Transylvania because that is where the lies about him started from.

NOTE: In parallel to these tails we also have the slavic stories that were circulating in the Romanian regions and the Balkans, depicting Dracula as a war hero that stood up to the turks. These are the stories that are made credible by the fact that Pope Pius II regarded Dracula as a valuable defender against the ottoman threat.